Fluid characterization is very important to the assessment of economic viability for a hydrocarbon-bearing reservoir formation. Some wireline tools such as Schlumberger's MDT (Modular Dynamic Tester) are used to sample formation fluids, store it in a set of bottles, and retrieve it to surface while keeping the fluid pressurized. Such samples are known as live fluids. These live fluids are then sent to an appropriate laboratory to be characterized. Characterization of the fluids may include composition analysis, fluid properties and phase behavior.
Understanding reservoir fluid phase behavior is key to proper planning and development of the respective fields and design of the production system. Understanding reservoir fluid phase behavior involves conducting a number of very important measurements on the fluid at realistic reservoir and production conditions. In most cases, changes in temperature (T) and pressure (P) of the formation fluid lead to phase changes, including phase separation (e.g., liquid-vapor, liquid-solid, liquid-liquid, vapor-liquid etc.), and phase recombination. For example, while most hydrocarbons exist as a single phase at initial reservoir conditions (i.e., composition, pressure, and temperature), they often undergo reversible (and possibly some irreversible) multi-phase changes due to pressure, composition and/or temperature reduction during production and flow to the surface facilities.
Liquid-Solid-Vapor phase boundaries are typically measured at a laboratory using state-of-the-art-technologies, such as Schlumberger's pressure-volume-temperature (PVT) unit coupled to Schlumberger's laser-based Solids Detection System (SDS) and Schlumberger's high-pressure microscope (HPM). Detailed descriptions of these state-of-the art technologies and their applications for the study of phase behavior and flow assurance of petroleum fluids have been published and are known to those of skill in the art.
However, the one trend in the industry is to perform more and more analysis of the formation and the formation fluid properties directly downhole to avoid the difficulties associated with sample preservation when lifted uphole and delays associated with sample transportation and analysis in a remote laboratory. Tools like Schlumberger's MDT can, for example, be retrofitted with a spectrometer module such as a Live Fluid Analyser or Gas Condensate Analyser in order to provide basic information on the fluid composition (Gas-to-oil ratio (GOR), water content, basic crackdown of hydrocarbon fractions (C1, C2-C5, C6+)). These measurements are performed by infrared (IR) absorption spectroscopy.
Nevertheless, current measurements of certain downhole characteristics do not facilitate full analysis of the formation and fluids, especially in situ. Fluorescence measurements downhole as discussed herein may be used to more fully characterize formations and formation fluids. In addition, U.S. Patent Application Publication Number 2004/0000636 assigned to Schlumberger Technology Corporation and invented by Oliver Mullins et al. discusses determining dew precipitation onset pressure in a sample located downhole in an oilfield reservoir, which may include measuring 1D fluorescence.
Further, while there has been some use of video imaging downhole in wireline tools, current technology is generally limited to applications related to production logging. Most current downhole imaging is dedicated to borehole wall imaging and has low spatial resolution (although commonly-owned U.S. patent application Ser. No. 11/204,134 discusses additional imaging capability). DHV International, for example, provides downhole video services to the oil and gas industry for diagnosis of borehole problems such as fishing out lost tools, mechanical inspection, and fluid entry surveys. There is room to improve methods and systems to more fully characterize formation fluids downhole.